(a) Field of the Invention
The present invention relates to liquid crystal display (LCD) device having a higher contrast ratio and, more particularly, to a LCD device including an interconnect line having a lower surface reflectance.
(b) Description of the Related Art
LCD devices are used in a variety of fields such as household use or medical use, and the number of fields in which the LCD devices are used are increasing. In general, the LCD device includes a pair of polarization plates disposed at the backlight side and the front side of a LCD panel and sandwiching therebetween the LCD panel. The polarization plates have respective polarization axes perpendicular to each other. By applying an electric field to the liquid crystal (LC) layer in the LCD device, the light components passing through the LC layer and polarization plate disposed at the front side are controlled for display of a desired image.
In the LCD device, the term “contrast ratio” is defined by a ratio of a brightness (black brightness) of the screen portion at which black is displayed to a brightness (white brightness) of the screen portion at which white is displayed. The contrast ratio is one of the indicators for evaluating the performances of the LCD device. In general, a higher white brightness and a lower black brightness achieve a higher contrast ratio, which provides an excellent display performance for the LCD device due to a clear distinction between the black and the white. For example, a specific LCD device for use in a medical instrument is desired to have a contrast ratio as high as 600 or above. In an in-plane switching mode (IPS) LCD device, which displays black upon application of no voltage thereto, a higher uniformity in the orientation of the liquid crystal molecules achieves a lower black brightness, thereby improving the contrast ratio.
It is noted that some of the light components incident onto the LCD device from the backlight source are scattered upside direction by the side of interconnect lines and electrodes, which may be sometimes referred to as collectively interconnect lines, thereby causing the change of polarized-state. The obliquely incident light have higher probability to be scattered when the interconnect lines, on which side the obliquely incident light is scattered, have larger thickness. Some of such scattered light are incident onto the polarization plate on the front side perpendicularly thereto. Thus, the polarization plate may pass the undesirable light components (scattered light) at black-state, to thereby lower the contrast ratio due to a resultant higher black brightness.
Patent Publication JP-A-11(1999)-337922 describes a technique for improving the contrast ratio of the LCD device, wherein the incident angles of the light components incident onto the LCD panel from the backlight source are limited within a specified angle. More specifically, the light components each having a specified angle or below with respect to the perpendicular line of the LCD panel are passed among all the light components incident onto the LCD panel from the backlight source, whereas the light components each having an angle higher than the specified angle are blocked. By reducing the light components passing obliquely within the LCD panel, the intensity of scattered light is reduced in the LCD panel to raise the contrast ratio.
If aluminum interconnect lines having a higher surface reflectance are provided in the pixel area through which light passes, especially in the IPS LCD device, it is likely to cause scattering of light. Accordingly, the restriction of the amount of light components obliquely passing in the LCD panel, as described in the publication, does not effectively prevent the light from being scattered by the side surfaces of the interconnect lines. Thus, a higher black brightness is caused to degrade the contrast ratio, whereby the improvement in the contrast ratio is not sufficient in the LCD devices.